MSSL Planetary Science Nuggets

Dione's thin oxygen exosphere

2 March 2012

Dione is a moon of Saturn, discovered in 1684 by the Italian astronomer Giovanni Cassini. Over 300 years later, planetary scientists including Andrew Coates and Geraint Jones from MSSL-UCL, have discovered that Dione has a weak exosphere near its surface (at planets with denser atmospheres, the exosphere is the outermost layer of its atmosphere). This exosphere is very very thin, about a million billionth of the Earth's atmospheric density. The work shows that the exosphere contains molecular oxygen - the same form of oxygen as in Earth's atmosphere. This gives us important information about how the atmosphere is produced.

When energetic (millions of degrees C) charged particles hit the icy surface of Dione they kick of molecules of water - this is called sputtering. Other energetic particles can then split up the hydrogen and oxygen in these water molecules in a process known as radiolysis. The hydrogen can escape from Dione but some of the molecular oxygen is trapped near to Dione forming the exosphere - and oxygen can be recycled via the surface, continuously replenishing the exosphere.

We know that this process also happens at Saturn's moon Rhea and also at Saturn's rings, where these charged particles sputter water molecules from the surfaces of the ring particles. Saturn isn't alone in this, however. Jupiter's icy moons Ganymede, Europa and Callisto, the targets of ESA's proposed JUICE mission for launch in 2022, also have exospheres produced by sputtering. This may therefore be a universal process, wherever an icy moon is bathed in a strong radiation environment, possibly even occuring in exoplanetary systems around other stars.